1. Field of the Invention
The present invention relates to a brushless motor used in e.g., a scanner motor and rotating a polygon mirror.
2. Description of the Related Art
A general image forming apparatus such as a laser printer, a digital copying machine, etc, has a polygon scanner for deflecting a light beam to perform a scanning operation. A high speed rotation has been recently required for this polygon scanner to increase an image forming speed of the image forming apparatus. A brushless motor having high performance is disposed in the polygon scanner to achieve this high speed rotation.
In the general polygon scanner having the brushless motor of this kind, for example, plural irregularities for generating a dynamic pressure are formed on an outer circumferential face of a fixed shaft fixed to a metallic housing. For example, these irregularities are constructed by herringbone grooves. A cylindrical rotating shaft is disposed outside this fixed shaft such that a predetermined bearing clearance is formed between the fixed shaft and the rotating shaft. Thus, a radial dynamic pressure air bearing is constructed and a rotary polygon mirror is further attached to this rotating shaft in a certain case.
The brushless motor has a magnet having 2n magnetic poles magnetized at an equal opening angle with respect to N and S poles and disposed in the rotating shaft. In this case, n is an even number equal to or greater than 2. The brushless motor further has cylindrical armatures each having three phases and opposed to this magnet and disposed in the housing. Normally, 3n/2 armatures are disposed in the brushless motor. In this brushless motor, the opening angle is set by a width of field magnetic poles arranged such that the field magnetic poles are alternately adjacent to each other. The armatures are arranged at an equal pitch such that the armatures do not overlap each other.
Concretely, the brushless motor has four magnetic poles, three armature coils and an opening angle of 90.degree.. In another system, the brushless motor has eight magnetic poles, six armature coils and an opening angle of 45.degree.. The armatures are mounted onto a printed board and the brushless motor is magnetized or driven by a driver, etc. disposed on the printed board in a predetermined way. Thus, the magnet is rotated so that the rotating shaft and the polygon mirror are rotated. For example, the brushless motor of this kind is shown in Japanese Patent Publication (KOKOKU) No. 58-26264.
However, this general brushless motor is constructed by 3n/2 armatures when the magnet is formed by 2n magnetic poles. For example, when the brushless motor has eight magnetic poles and six armature coils, a magnetizing frequency of the brushless motor is increased when the polygon mirror is rotated at a high speed equal to or higher than 20,000 rpm. Therefore, driving efficiency of a control circuit for controlling an operation of the brushless motor is reduced so that an electric current flowing through the brushless motor is increased. Further, an iron loss of the brushless motor is increased so that a temperature of the brushless motor is increased.
Namely, when the brushless motor is constructed by eight magnetic poles and six armature coils, the number of magnetic poles is large so that the magnetizing frequency is necessarily set to a high frequency and driving efficiency of the control circuit is therefore reduced. The iron loss is increased in accordance with the second power of a frequency. Accordingly, when the magnetizing frequency is increased, the iron loss is necessarily increased so that a temperature of the brushless motor is increased.
In contrast to this, when the brushless motor is constructed by four magnetic poles and three armature coils and an inductive voltage constant equal to that provided by six armature coils is obtained, the number of turns per one coil is twice or more that in the case of the six armature coils. Accordingly, a winding coil must be thin to secure a space similar to that provided by the eight magnetic poles and the six armature coils. However, when the winding coil is set to be thin, electric resistance of the brushless motor is increased so that a copper loss of the brushless motor is caused and a temperature of the brushless motor is thereby increased. Accordingly, in each of the above cases, it is impossible to sufficiently cope with a high speed rotation of the brushless motor.
Further, in this brushless motor, it is necessary to correct the balance of a rotating body of the rotating shaft, the polygon mirror, etc. with high accuracy so as to rotate the polygon scanner, etc. at a high rotational speed with low vibration. Therefore, a ring for balance correction is disposed in the polygon scanner and a circumferential groove is processed and formed in a member for fixing the magnet thereto so that cost of the brushless motor is increased.
Further, the housing of the brushless motor is formed by a metal so that it is necessary to electrically connect the housing to the ground when this brushless motor is assembled into a copying machine, etc. When a constructional member of the copying machine is formed by a metallic material, there is no problem when the brushless motor is assembled into the copying machine. However, when this constructional member is formed by an insulating material such as resin cheaply manufactured, one wire end portion of the constructional member is generally fixed to a housing portion by a screw. Thereafter, another wire end portion is connected to a metallic frame of the copying machine. Therefore, an attaching operation of the brushless motor is complicated so that assembly cost of the brushless motor is very increased.